Microtiter plates are often used to incubate and culture a heterologous library of cells, or strains. For example, a library of 96 strains of cells can be compared in a single 96-well microtiter plate by culturing a different strain in each well. Thus, differences between the cells (e.g., growth rate, carbon utilization, population density, viability, protein production rate, and resistance to antibiotics) can be observed in a small format. In conventional microtiter plates, a fixed amount of carbon and other nutrients are typically batch provided to the cultured cells. As the cells grow, one or more of the nutrients can rapidly be limited or exhausted and cause the cells to slow or halt growth before differences between the cells can be observed. Thus, conventional microtiter plates are not well suited for screening libraries of cells.
To address issues with carbon limitation in conventional microtiter plates, a slow-release system for glucose delivery, the “Feedbead®” technology (Jeude et al., Biotechnol Bioeng 95:433-445, 2006), was developed for preparing pre-cultures. Feedbead® discs (AdolfKühner AG) are silicone discs embedded with sugar. Feedbead® discs, however, have several limitations. First, only relatively small amounts of sugar can be packed into such solid phases. Second, the total amount of sugar available to the culture is limited by the geometry of the disc (the discs do not contain enough carbon for sustained production). Third, the sugar release rate from such a solid phase is fastest at the beginning of the cultivation, when the amount of cells is lowest and the risk for overflow metabolism is highest. Fourth, the approach has limited scalability due to, e.g., the amount of sugar that can be packed into the beads and the lack of means to accurately control the sugar release. Finally, the presence of the Feedbead® discus or other immobilized controlled release systems (e.g., coatings, attachments) in the well interferes with bulk pipetting in screening applications. For these reasons, the technique has mostly been limited to pre-cultures (Huber et al., Biotechnol Bioeng 103:1095-1102, 2009).